1. Field of the Invention
The invention relates to an improved process for the preparation of high activity magnesium and titanium containing complex catalysts for the polymerization of ethylene in a gas phase process to produce polymers having a density of .gtoreq.0.91 to .ltoreq.0.97; a melt flow ratio of .gtoreq.22 to .ltoreq.32; and a bulk density of 14 to 32 lbs/ft.sup.3.
2. Description of the Prior Art
To be commercially useful in a gas phase process, such as the fluid bed process of U.S. Pat. Nos. 3,709,853; 4,003,712 and 4,011,382, Canadian Pat. No. 991,798 and Belgium Pat. No. 839,380, the catalyst employed must be a high activity catalyst, that is, it must have a level of productivity of .gtoreq.50,000, and preferably of .gtoreq.100,000 pounds of polymer per pound of primary metal in the catalyst. This is so because such gas phase processes usually do not employ any catalyst residue removing procedures. Thus, the catalyst residue in the polymer must be so small that it can be left in the polymer without causing any undue problems in the hands of the resin fabricator and/or ultimate consumer. Where a high activity catalyst is successfully used in such fluid bed processes, the transition metal content of the resin is of the order of .ltoreq.20 parts per million (ppm) at a productivity level of .gtoreq.50,000. Low catalyst residue contents are also important where the catalyst is made with chlorine containing materials such as the titanium, magnesium and/or aluminum chlorides used in some so-called Ziegler or Ziegler-Natta catalysts. High residual chlorine values in a molding resin will cause pitting and corrosion on the metal surfaces of the molding devices. Molding grade resins having Cl residues of the order of .gtoreq.200 ppm are not commercially useful.
U.S. Pat. Nos. 3,922,322 and 4,035,560 disclose the use of several Ti and Mg containing catalysts for the manufacture of granular ethylene polymers in a gas phase fluid bed process under a pressure of &lt;1000 psi. The use of these catalysts in these processes, however, has significant disadvantages. The catalysts of U.S. Pat. No. 3,922,322 provide polymers having a very high catalyst residue content, i.e., about 100 ppm of Ti and greater than about 300 ppm Cl, according to the working example of this patent. Further, as disclosed in the working example of U.S. Pat. No. 3,922,322, the catalyst is used in the form of a prepolymer, and very high volumes of the catalyst composition must be fed to the reactor relative to the volume of polymer made in the reactor. The preparation and use of this catalyst thus requires the use of relatively large sized equipment for the manufacture, storage and transporting of the catalyst.
The catalysts of U.S. Pat. No. 4,035,560 also provides polymers having high catalyst residues, and the catalyst compositions are apparently pyrophoric because of the types and amounts of reducing agents employed in such catalysts.
U.S. patent application Ser. No. 892,325, filed Mar. 31, 1978, now abandoned and refiled as Ser. No. 014,414 on Feb. 27, 1979, in the names of F. J. Karol et al., and entitled "Preparation of Ethylene Copolymers in Fluid Bed Reactor", now U.S. Pat. No. 4,302,566, discloses that ethylene copolymers, having a density of 0.91 to 0.96, a melt flow ratio of .gtoreq.22 to .ltoreq.32 and a relatively low residual catalyst content can be produced in granular form, at relatively high productivities, if the monomer(s) are polymerized in a gas phase process with a specific high activity Mg-Ti containing complex catalyst which is blended with an inert carrier material. The granular polymers thus produced have excellent physical properties which allow them to be used in a broad range of molding applications.
U.S. patent application Ser. No. 892,037 filed on Mar. 31, 1978, now abandoned and refiled as Ser. No. 014,412 on Feb. 27, 1979, and refiled again as Ser. No. 249,447 on Mar. 31, 1981, in the names of B. E. Wagner et al. and entitled "Polymerization Catalyst, Process for Preparing, And Use For Ethylene Homopolymerization", now U.S. Pat. No. 4,395,359, discloses that ethylene homopolymers having a density range of 0.958 to 0.972 and a melt flow ratio of .gtoreq.22 to .ltoreq.32 and which have a relatively low residual catalyst residue can be produced at relatively high productivities for commercial purposes by a low pressure gas phase process if the ethylene is homopolymerized in the presence of a high activity magnesium-titanium complex catalyst blended with an inert carrier material.
U.S. patent application Ser. No. 892,322 filed on Mar. 31, 1978, now abandoned and refiled as Ser. No. 012,720 on Feb. 16, 1979, in the names of G. L. Goeke et al. and entitled "Impregnated Polymerization Catalyst, Process For Preparing, and Use For Ethylene Copolymerization", now U.S. Pat. No. 4,302,565, discloses that ethylene copolymers having a density of about 0.91 to 0.94 and a melt flow ratio of .gtoreq.22 to .ltoreq.32 and which have a relatively high bulk density and which provide films of good clarity can be produced at relatively high productivities for commercial purposes by a gas phase process if the ethylene is copolymerized with one or more C.sub.3 to C.sub.8 alpha olefins in the presence of a high activity magnesium-titanium complex catalyst prepared under specific activation conditions with an organoaluminum compound and impregnated in a porous inert carrier material.
However, the preparation of the impregnated catalyst precursor as taught in U.S. Pat. No. 4,302,565, can be difficult to control and the carrier material used for the impregnation can be of variable composition. If considerable care is not taken, variable catalyst performance can occur. Since polymer morphology appears to be dependent on the morphology of the carrier used for the catalyst, total flexibility and control of polymer particle characteristics is, at times, not possible.
U.S. patent application Ser. No. 974,013 filed on Dec. 28, 1978 now abandoned and refiled as Ser. No. 095,010 on Nov. 28, 1979, in the names of A. D. Hamer et al. and entitled "Spheroidal Polymerization Catalyst, Process for Preparing, and Use for Ethylene Polymerization," now U.S. Pat. No. 4,293,673 discloses that ethylene polymers having a wide density range of about .gtoreq.0.91 to .ltoreq.0.97, a bulk density of about 18 to 32 lbs/ft.sup.3, a melt flow ratio of .gtoreq.22 to .ltoreq.32, and which are of controlled particle shape and size, and which have a relatively low residual titanium content can be produced at relatively high productivities for commercial purposes by a gas phase process if the ethylene is homopolymerized, or copolymerized with one or more C.sub.3 to C.sub.8 alpha olefins, in the presence of a high activity magnesium-titanium complex catalyst prepared by spray drying a magnesium-titanium containing precursor composition from a slurry or solution in an electron donor solvent and subsequently activating such spray dried precursor composition under specific activation conditions with an organoaluminum compound.
The catalyst preparation disclosure in U.S. Pat. No. 4,302,565 teaches that first a catalyst precursor composition is prepared from a titanium compound, a magnesium compound and an electron donor solvent; a carrier material previously treated with an organoaluminum compound is then impregnated with a THF solution of the precursor composition; and finally the supported precursor composition is treated with an activator compound in one or more steps.
In more detail, U.S. Pat. No. 4,302,565 teaches that preferably the precursor composition is formed by dissolving the titanium compound and the magnesium compound in the electron donor solvent at a temperature of about 20.degree. C. up to the boiling point of the electron donor solvent. The titanium compound can be added to the electron donor solvent before or after the addition of the magnesium compound, or concurrent therewith. The dissolution of the titanium compound and the magnesium compound can be facilitated by stirring, and in some instances by refluxing these two compounds in the electron donor solvent. After the titanium compound and the magnesium compound are dissolved, the precursor composition may be isolated in some manner such as crystallization or by precipitation with a C.sub.5 to C.sub.8 aliphatic or aromatic hydrocarbon such as hexane, isopentane or benzene and subsequently dried.
The dried precursor composition is in the form of fine, free flowing particles having an average particle size of about 10 to 100 microns and a bulk density of about 18 to 33 pounds per cubic foot.
The precursor composition is then impregnated, in a weight ratio of about 0.033 to 1, and preferably about 0.1 to 0.33, parts of the precursor composition into one part by weight of the carrier material.
The impregnation of the support with the precursor composition may be accomplished by dissolving the precursor composition in the electron donor solvent, and by then admixing the support with the dissolved precursor composition so as to allow the precursor composition to impregnate the support. The solvent is then removed by drying at temperatures of .ltoreq.70.degree. C.
The support is preferably impregnated with the precursor composition by adding the support to a solution of the chemical raw materials used to form the precursor composition in the electron donor solvent, without isolating the precursor composition from such solution. The excess electron donor solvent is then removed by drying, or decanting and drying, at temperatures of .ltoreq.70.degree. C.
In order to prepare a useful catalyst, the supported precursor composition must be treated with sufficient activator compound to transform the Ti atoms in the precursor composition to an active state. However, it is necessary to conduct the activation in such a way that, at least the final activation stage is conducted in the absence of solvent so as to avoid the need for drying the fully active catalyst to remove solvent therefrom. Two procedures have been developed to accomplish this result.
In one procedure, the precursor composition is completely activated, outside the reactor, in the absence of solvent, by dry blending the impregnated precursor composition with the activator compound. In this dry blending procedure the activator compound is used while impregnated in a carrier material. In this procedure the fully activated precursor composition is prepared without having to heat the composition above 50.degree. C. prior to feeding it to the polymerization reactor.
In the second, and preferred of such prior art catalyst activation precedures, the precursor composition is partially activated outside the polymerization reactor with enough activator compound so as to provide a partially activated precursor composition which has an activator compound/Ti molar ratio of about &lt;0 to &gt;10:1 and preferably of about 4 to 8:1. This partial activation reaction is preferably carried out in a hydrocarbon solvent slurry followed by drying of the resulting mixture, to remove the solvent, at temperatures between 20.degree. to 80.degree. C. and preferably of 50.degree. to 70.degree. C. The resulting product is a free-flowing, solid particulate material which can be readily fed to the polymerization reactor. The partially activated and impregnated precursor composition is fed to the polymerization reactor where the activation is completed with additional activator compound which can be the same or a different compound.
The above-described catalyst preparation of U.S. Pat. No. 4,302,565 requires a minimum of two drying stages: (1) excess electron donor solvent removal from the impregnated composition; and (2) hydrocarbon solvent removal from the partially activated precursor composition.
The above-described preferred catalyst preparation of U.S. Pat. No. 4,302,565 requires a minimum of at least three drying stages: (1) hydrocarbon solvent removal from the treated carrier; (2) excess electron donor solvent removal from the impregnated precursor composition; and (3) hydrocarbon solvent removal from the partially activated impregnated precursor composition. It may be desirable to isolate the precursor composition, in which case an additional drying step is required to remove excess electron donor solvent. The number of drying steps is a critical limitation to the commercial attractiveness of these catalyst compositions.
Drying at each stage in the catalyst preparation plays an important role in determining the final catalyst properties since the drying conditions determine the final electron donor and activator content. Furthermore, as a result of the porosity of the inert carrier material utilized in the preferred embodiments of U.S. Pat. No. 4,302,565, as well as other considerations, the drying times of some of the stages in commercial catalyst preparation processes have exceeded 15 hours. Even with extensive expenditures for drying enhancement, such as imposing vacuums, the drying stages follow the time constraints imposed by the classical behavior of the drying of porous solid slurries. The drying rate typically falls into two regions: constant rate region and falling rate region. In the constant rate region, the solid surface is saturated with evaporating liquid and the rate of drying is controlled by the rate of heat transfer to the evaporating surface. Since the rate of mass transfer balances the rate of heat transfer, the temperature of the saturated surface remains constant. When the surface concentration falls below the saturation point, the drying rate starts to fall and the drying rate is dictated by the ability of the liquid to diffuse to the surface. This is the falling rate region and the solid surface temperature in this period starts to increase because of the diffusion limitation. Movement within the solid results from a concentration gradient which is dependent upon the characteristics of the solid. Of course, the solid temperature asymptotically reaches the vessel wall temperature as the liquid concentration approaches zero.
Removal of excess electron donor solvent from the precursor composition is especially critical. Excess electron donor solvent is essential in order to completely dissolve the magnesium and titanium compounds. However, the patent literature teaches that excess electron donor solvent should be removed prior to activation of a catalyst precursor. For example, U.S. Pat. No. 4,120,883 to H. Sakurai et al. discloses that whenever organomagnesium complexes are employed as catalysts, ethers should not be employed as solvents for the synthesis and, if used, should be removed prior to activation.
Furthermore, in the prior art it was recognized that electron donor solvents such as tetrahydrofuran and activator compounds such as the aluminum alkyls react to form 1:1 adducts (R. N. Haszeldine et al., Polymer, 14,215 (1973)). Since formation of these adducts might destroy the beneficial effects of the activator compound, U.S. Pat. Nos. 4,302,565, 4,302,566 and 4,395,359 specifically described the need to minimize the electron donor content of the precursor composition and to partially activate the precursor in a hydrocarbon diluent in order to avoid the presence of excess electron donor solvent.